The beta-amyloid precursor protein (beta APP) is a widely expressed integral membrane protein that is proteolytically processed to yield several secreted derivatives, including soluble APP (APPs), the 4-kDa amyloid beta-peptide (A beta), and a related 3-kDa peptide (p3). To understand beta APP trafficking and processing, we analyzed the sorting of beta APP in Madin-Darby canine kidney (MDCK) cells, an epithelial cell known to possess physiologically distinct apical and basolateral plasma membranes. Processing of beta APP resulted in highly polarized secretion of APPs. More than 90% of APPs was detected in the basolateral compartment, and less than 10% was found in the apical compartment. This was associated with a preferential localization of beta APP on the basolateral cell surface. Activation of protein kinase C, which is known to enhance the secretion of APPs, did not change the polarity of APPs release but significantly increased the amount secreted. A beta and p3 peptides were also secreted predominantly basolaterally. In addition, MDCK cells secreted a truncated form of A beta beginning at Arg-5. These data show that the proteolytic processing products of beta APP undergo polarized secretion. Moreover, the results suggest that the amyloidogenic A beta peptide is generated following the polarized sorting of beta APP. The polarized basolateral secretion of A beta in these epithelial cells provides a potential mechanism for the accumulation of A beta in the abluminal basement membrane of brain microvessels during Alzheimer disease.